Noninvasive Raman spectroscopy of rat tibiae: approach to in vivo assessment of bone quality

We report on in vivo noninvasive Raman spectroscopy of rat tibiae using robust fiber-optic Raman probes and holders designed for transcutaneous Raman measurements in small animals. The configuration allows placement of multiple fibers around a rat leg, maintaining contact with the skin. Bone Raman data are presented for three regions of the rat tibia diaphysis with different thicknesses of overlying soft tissue. The ability to perform in vivo noninvasive Raman measurement and evaluation of subtle changes in bone composition is demonstrated with rat leg phantoms in which the tibia has carbonated hydroxylapatite, with different carbonate contents. Our data provide proof of the principle that small changes in bone composition can be monitored through soft tissue at anatomical sites of interest in biomedical studies.

Wrapping and dispersion of multiwalled carbon nanotubes improves electrical conductivity of protein-nanotube composite biomaterials

Composites of extracellular matrix proteins reinforced with carbon nanotubes have the potential to be used as conductive biopolymers in a variety of biomaterial applications. In this study, the effect of functionalization and polymer wrapping on the dispersion of multiwalled carbon nanotubes (MWCNT) in aqueous media was examined. Carboxylated MWCNT were wrapped in either Pluronic(®) F127 or gelatin. Raman spectroscopy and X-ray photoelectron spectroscopy showed that covalent functionalization of the pristine nanotubes disrupted the carbon lattice and added carboxyl groups. Polymer and gelatin wrapping resulted in increased surface adsorbed oxygen and nitrogen, respectively. Wrapping also markedly increased the stability of MWCNT suspensions in water as measured by settling time and zeta potential, with Pluronic(®)-wrapped nanotubes showing the greatest effect. Treated MWCNT were used to make 3D collagen-fibrin-MWCNT composite materials. Carboxylated MWCNT resulted in a decrease in construct impedance by an order of magnitude, and wrapping with Pluronic(®) resulted in a further order of magnitude decrease. Functionalization and wrapping also were associated with maintenance of fibroblast function within protein-MWCNT materials. These data show that increased dispersion of nanotubes in protein-MWCNT composites leads to higher conductivity and improved cytocompatibility. Understanding how nanotubes interact with biological systems is important in enabling the development of new biomedical technologies.

Age-specific profiles of tissue-level composition and mechanical properties in murine cortical bone

There is growing evidence that bone composition and tissue-level mechanical properties are significant determinants of skeletal integrity. In the current study, Raman spectroscopy and nanoindentation testing were co-localized to analyze tissue-level compositional and mechanical properties in skeletally mature young (4 or 5 months) and old (19 months) murine femora at similar spatial scales. Standard multivariate linear regression analysis revealed age-dependent patterns in the relationships between mechanical and compositional properties at the tissue scale. However, changes in bone material properties with age are often complex and nonlinear, and can be missed with linear regression and correlation-based methods. A retrospective data mining approach was implemented using non-linear multidimensional visualization and classification to identify spectroscopic and nanoindentation metrics that best discriminated bone specimens of different age-classes. The ability to classify the specimens into the correct age group increased by using combinations of Raman and nanoindentation variables (86-96% accuracy) as compared to using individual measures (59-79% accuracy). Metrics that best classified 4 or 5 month and 19 month specimens (2-age classes) were mineral to matrix ratio, crystallinity, modulus and plasticity index. Metrics that best distinguished between 4, 5 and 19 month specimens (3-age classes) were mineral to matrix ratio, crystallinity, modulus, hardness, cross-linking, carbonate to phosphate ratio, creep displacement and creep viscosity. These findings attest to the complexity of mechanisms underlying bone tissue properties and draw attention to the importance of considering non-linear interactions between tissue-level composition and mechanics that may work together to influence material properties with age. The results demonstrate that a few non-linearly combined compositional and mechanical metrics provide better discriminatory information than a single metric or a single technique.

CARBON BLACK FOR RACING AND MOTORCYCLE TIRE TREADS. PERFORMANCE MODEL DEVELOPMENT

Racing tires and motorcycle tires present individual segments of the tire market. For instance, while the average life of car and truck tires is 50 000 miles, the average life of race tires is 100 miles. Because tires play a critical role in a race, technical demands to assure safety and performance are growing. Similarly, tires have a large influence on safety, handling/grip, and performance of the rapidly growing world fleet of motorcycles, due to the fact of only two wheels being in contact with the ground. Thus, the common feature of both market segments is that the typical tire compromise of wear, rolling resistance, and traction is strongly weighted toward traction. Most of the recent efforts of rubber scientists have been directed toward lowering rolling resistance of the tread compounds, which left a certain void in the science of compounding for racing and motorcycle treads. Particularly, the industrial assortment of polymers and fillers used for motorcycle treads is commonly different from that used for car or truck treads, but it is not known how the filler properties affect the hysteresis–stiffness compromise. The objective of this study is to evaluate the effects of the carbon black characteristics on the important properties of a typical racing and motorcycle tire tread compound. More than 50 individual carbon blacks were mixed in a SBR formulation. The acquired data were statistically analyzed, and a linear multiple regression model was developed to relate rubber properties (responses), such as static modulus, complex dynamic modulus, hysteresis, and viscosity to the key carbon black characteristics (variables) of surface area, structure, aggregate size distribution, and surface activity. Prediction profiles created from the model demonstrate rubber performance limits for the range of carbon blacks tested, and indicate the niches to provide required combinations of the rubber properties.

Biomedical tissue phantoms with controlled geometric and optical properties for Raman spectroscopy and tomography

To support the translation of Raman spectroscopy into clinical applications, synthetic models are needed to accurately test, optimize and validate prototype fiber optic instrumentation. Synthetic models (also called tissue phantoms) are widely used for developing and testing optical instrumentation for diffuse reflectance, fluorescence, and Raman spectroscopies. While existing tissue phantoms accurately model tissue optical scattering and absorption, they do not typically model the anatomic shapes and chemical composition of tissue. Because Raman spectroscopy is sensitive to molecular composition, Raman tissue phantoms should also approximate the bulk tissue composition. We describe the fabrication and characterization of tissue phantoms for Raman tomography and spectroscopy. These phantoms have controlled chemical and optical properties, and also multilayer morphologies which approximate the appropriate anatomic shapes. Tissue phantoms were fabricated to support on-going Raman studies by simulating the human wrist and rat leg. Surface meshes (triangle patch models) were generated from computed tomography (CT) images of a human arm and rat leg. Rapid prototyping was used to print mold templates with complex geometric patterns. Plastic casting techniques used for movie special effects were adapted to fabricate molds from the rapid prototypes, and finally to cast multilayer gelatin tissue phantoms. The gelatin base was enriched with additives to model the approximate chemistry and optical properties of individual tissue layers. Additional studies were performed to determine optimal casting conditions, phantom stability, layer delamination and chemical diffusion between layers. Recovery of diffuse reflectance and Raman spectra in tissue phantoms varied with probe placement. These phantoms enable optimization of probe placement for human or rat studies. These multilayer tissue phantoms with complex geometries are shown to be stable, with minimal layer delamination and chemical diffusion.

Polymer-capped fiber-optic Raman probe for non-invasive Raman spectroscopy

Advances in fiber optic probe design are moving Raman spectroscopy into the clinic, although there remain important practical problems. While much effort has been devoted to minimizing Raman and fluorescence background from fibers, less attention has been given to the need to generate reference Raman signals that can correct for variations in tissue albedo, which is important in quantifying changes in tissue composition. To address this shortcoming, we have developed a fiber optic probe that incorporates a fluorinated ethylene-propylene copolymer (FEP) cap at the end of each excitation fiber. Transmission of laser light through the transparent cap generates a 732 cm(-1) Raman band whose intensity scales linearly with the laser power delivered to the tissue of interest. In our first design, the FEP cap functions as a waveguide with only a small insertion loss (~5%). Laser transmission through 1 mm of the polymer is sufficient to generate a usable reference Raman signal. We show the application of the probe to quantitative non-invasive Raman spectroscopy of animal tissues using rat leg phantoms as models. Ex-vivo Raman spectroscopy of excised rat tibia supports the use of the probe for spectroscopy of various tissues. These results provide proof of principle that the Raman probe can be used in multiple spectroscopic applications.

Solid-state NMR spectroscopy provides atomic-level insights into the dehydration of cartilage

An atomic-level insight into the functioning of articular cartilage would be useful to develop prevention strategies and therapies for joint diseases such as osteoarthritis. However, the composition and structure of cartilage and their relationship to its unique mechanical properties are quite complex and pose tremendous challenges to most biophysical techniques. In this study, we present an investigation of the structure and dynamics of polymeric molecules of articular cartilage using time-resolved solid-state NMR spectroscopy during dehydration. Full-thickness cartilage explants were used in magic-angle spinning experiments to monitor the structural changes of rigid and mobile carbons. Our results reveal that the dehydration reduced the mobility of collagen amino acid residues and carbon sugar ring structures in glycosaminoglycans but had no effect on the trans-Xaa-Pro conformation. Equally interestingly, our results demonstrate that the dehydration effects are reversible, and the molecular structure and mobility are restored upon rehydration.

Raman assessment of bone quality

BACKGROUND

Progress in the diagnosis and prediction of fragility fractures depends on improvements to the understating of the compositional contributors of bone quality to mechanical competence. Raman spectroscopy has been used to evaluate alterations to bone composition associated with aging, disease, or injury.

QUESTIONS/PURPOSES

In this survey we will (1) review the use of Raman-based compositional measures of bone quality, including mineral-to-matrix ratio, carbonate-to-phosphate ratio, collagen quality, and crystallinity; (2) review literature correlating Raman spectra with biomechanical and other physiochemical measurements and with bone health; and (3) discuss prospects for ex vivo and in vivo human subject measurements.

METHODS

ISI Web of Science was searched for references to bone Raman spectroscopy in peer-reviewed journals. Papers from other topics have been excluded from this review, including those on pharmaceutical topics, dental tissue, tissue engineering, stem cells, and implant integration.

RESULTS

Raman spectra have been reported for human and animal bone as a function of age, biomechanical status, pathology, and other quality parameters. Current literature supports the use of mineral-to-matrix ratio, carbonate-to-phosphate ratio, and mineral crystallinity as measures of bone quality. Discrepancies between reports arise from the use of band intensity ratios rather than true composition ratios, primarily as a result of differing collagen band selections.

CONCLUSIONS

Raman spectroscopy shows promise for evaluating the compositional contributors of bone quality in ex vivo specimens, although further validation is still needed. Methodology for noninvasive in vivo assessments is still under development.

Repeated freeze-thawing of bone tissue affects Raman bone quality measurements

The ability to probe fresh tissue is a key feature to biomedical Raman spectroscopy. However, it is unclear how Raman spectra of calcified tissues are affected by freezing. In this study, six transverse sections of femoral cortical bone were subjected to multiple freeze∕thaw cycles and probed using a custom Raman microscope. Significant decreases were observed in the amide I and amide III bands starting after two freeze thaw cycles. Raman band intensities arising from proline residues of frozen tissue appeared consistent with fresh tissue after four cycles. Crystallinity values of bone mineral diminished slightly with freezing and were noticeable after only one freezing. Mineral carbonate levels did not deviate significantly with freezing and thawing. The authors recommend freezing and thawing bone tissue only once to maintain accurate results.

Fiber-optic Raman spectroscopy of joint tissues

In this study, we report adaptation of Raman spectroscopy for arthroscopy of joint tissues using a custom-built fiber-optic probe. Differentiation of healthy and damaged tissue or examination of subsurface tissue, such as subchondral bone, is a challenge in arthroscopy because visual inspection may not provide sufficient contrast. Discrimination of healthy versus damaged tissue may be improved by incorporating point spectroscopy or hyperspectral imaging into arthroscopy where the contrast is based on the molecular structure or chemical composition. Articular joint surfaces of knee cadaveric human tissue and tissue phantoms were examined using a custom-designed Raman fiber-optic probe. Fiber-optic Raman spectra were compared against reference spectra of cartilage, subchondral bone and cancellous bone collected using Raman microspectroscopy. In fiber-optic Raman spectra of the articular surface, there was an effect of cartilage thickness on recovery of signal from subchondral bone. At sites with intact cartilage, the bone mineralization ratio decreased but there was a minimal effect in the bone mineral chemistry ratios. Tissue phantoms were prepared as experimental models of the osteochondral interface. Raman spectra of tissue phantoms suggested that optical scattering of cartilage has a large effect on the relative cartilage and bone signal. Finite element analysis modeling of light fluence in the osteochondral interface confirmed experimental findings in human cadaveric tissue and tissue phantoms. These first studies demonstrate the proof of principle for Raman arthroscopic measurement of joint tissues and provide a basis for future clinical or animal model studies.

Minor distortions with major consequences: correcting distortions in imaging spectrographs

Projective transformation is a mathematical correction (implemented in software) used in the remote imaging field to produce distortion-free images. We present the application of projective transformation to correct minor alignment and astigmatism distortions that are inherent in dispersive spectrographs. Patterned white-light images and neon emission spectra were used to produce registration points for the transformation. Raman transects collected on microscopy and fiber-optic systems were corrected using established methods and compared with the same transects corrected using the projective transformation. Even minor distortions have a significant effect on reproducibility and apparent fluorescence background complexity. Simulated Raman spectra were used to optimize the projective transformation algorithm. We demonstrate that the projective transformation reduced the apparent fluorescent background complexity and improved reproducibility of measured parameters of Raman spectra. Distortion correction using a projective transformation provides a major advantage in reducing the background fluorescence complexity even in instrumentation where slit-image distortions and camera rotation were minimized using manual or mechanical means. We expect these advantages should be readily applicable to other spectroscopic modalities using dispersive imaging spectrographs.

Natural-abundance 43Ca solid-state NMR spectroscopy of bone

Structural information about the coordination environment of calcium present in bone is highly valuable in understanding the role of calcium in bone formation, biomineralization, and bone diseases like osteoporosis. While a high-resolution structural study on bone has been considered to be extremely challenging, NMR studies on model compounds and bone minerals have provided valuable insight into the structure of bone. Particularly, the recent demonstration of (43)Ca solid-state NMR experiments on model compounds is an important advance in this field. However, application of (43)Ca NMR is hampered due to the low natural-abundance and poor sensitivity of (43)Ca. In this study, we report the first demonstration of natural-abundance (43)Ca magic angle spinning (MAS) NMR experiments on bone, using powdered bovine cortical bone samples. (43)Ca NMR spectra of bovine cortical bone are analyzed by comparing to the natural-abundance (43)Ca NMR spectra of model compounds including hydroxyapatite and carbonated apatite. While (43)Ca NMR spectra of hydroxyapatite and carbonated apatite are very similar, they significantly differ from those of cortical bone. Raman spectroscopy shows that the calcium environment in bone is more similar to carbonated apatite than hydroxyapatite. A close analysis of (43)Ca NMR spectra reveals that the chemical shift frequencies of cortical bone and 10% carbonated apatite are similar but the quadrupole coupling constant of cortical bone is larger than that measured for model compounds. In addition, our results suggest that an increase in the carbonate concentration decreases the observed (43)Ca chemical shift frequency. A comparison of experimentally obtained (43)Ca MAS spectra with simulations reveal a 3:4 mol ratio of Ca-I/Ca-II sites in carbonated apatite and a 2.3:3 mol ratio for hydroxyapatite. 2D triple-quantum (43)Ca MAS experiments performed on a mixture of carbonated apatite and the bone protein osteocalcin reveal the presence of protein-bound and free calcium sites, which is in agreement with a model developed from X-ray crystal structure of the protein.

Development of non-invasive Raman spectroscopy for in vivo evaluation of bone graft osseointegration in a rat model

The use of bone structural allografts for reconstruction following tumor resection is widespread, although successful incorporation and regeneration remain uncertain. There are few non-invasive methods to fully assess the progress of graft incorporation. Computed tomography and MRI provide information on the morphology of the graft/host interface. Limited information is also available from DXA and ultrasound. Only few techniques can provide information on the metabolic status of the graft, such as the mineral and matrix composition of the regenerated tissue that may provide early indications of graft success or failure. To address this challenge, we discuss here the implementation of Raman spectroscopy for in vivo assessment of allograft implantation in a rat model. An array of optical fibers was developed to allow excitation and collection of Raman spectra through the skin of rat at various positions around the rat's tibia. The system is calibrated against locally constructed phantoms that mimic the morphology, optics and spectroscopy of the rat. The system was evaluated by carrying out transcutaneous Raman measurement on rat. Bone mineral and matrix Raman bands are successfully recovered. This new technology provides a non-invasive method for in vivo monitoring of bone graft osseointegration.

Exogenous cholesterol transport in rabbit plasma lipoproteins

1. The appearance of exogenous cholesterol in free cholesterol and ester cholesterol of plasma chylomicra, very-low-density (VLD), low-density (LD) and high-density (HD) lipoproteins was studied in unanaesthetized rabbits after ingestion of a meal containing 5% fat and 0.08% [(3)H]cholesterol. 2. The specific radioactivity of ester cholesterol of VLD lipoproteins reached the highest value of any lipoprotein fraction and for each lipoprotein it increased at a faster rate and reached a higher maximum than that of free cholesterol; the maximum in VLD lipoproteins occurred later than in chylomicra. 3. The pattern of appearance of exogenous cholesterol in chylomicra and VLD lipoproteins of plasma was similar to the pattern previously observed in lymph. The specific radioactivity of ester cholesterol in plasma VLD lipoproteins was higher than that in chylomicra in spite of a larger pool size and dilution of cholesteryl esters from VLD lipoproteins produced by the liver. These results support the concept that during absorption the major portion of exogenous cholesterol is transported in VLD lipoproteins as ester cholesterol. 4. The specific radioactivity of ester cholesterol of chylomicra and VLD lipoproteins increased at a faster rate than that of LD and HD lipoproteins. However, the rate of increase and the absolute values of the specific radioactivity in LD and HD lipoproteins were identical. Since cholesteryl esters are thought not to exchange between lipoproteins, this observation supports the hypothesis that a result of VLD lipoprotein and chylomicron metabolism is the formation of LD and HD lipoproteins. 5. Results in vivo showed that the free cholesterol of individual plasma lipoproteins does not equilibrate within a period of 24h.

Quantitative polarized Raman spectroscopy in highly turbid bone tissue

Polarized Raman spectroscopy allows measurement of molecular orientation and composition and is widely used in the study of polymer systems. Here, we extend the technique to the extraction of quantitative orientation information from bone tissue, which is optically thick and highly turbid. We discuss multiple scattering effects in tissue and show that repeated measurements using a series of objectives of differing numerical apertures can be employed to assess the contributions of sample turbidity and depth of field on polarized Raman measurements. A high numerical aperture objective minimizes the systematic errors introduced by multiple scattering. We test and validate the use of polarized Raman spectroscopy using wild-type and genetically modified (oim/oim model of osteogenesis imperfecta) murine bones. Mineral orientation distribution functions show that mineral crystallites are not as well aligned (p<0.05) in oim/oim bones (28+/-3 deg) compared to wild-type bones (22+/-3 deg), in agreement with small-angle X-ray scattering results. In wild-type mice, backbone carbonyl orientation is 76+/-2 deg and in oim/oim mice, it is 72+/-4 deg (p>0.05). We provide evidence that simultaneous quantitative measurements of mineral and collagen orientations on intact bone specimens are possible using polarized Raman spectroscopy.

Time-resolved dehydration-induced structural changes in an intact bovine cortical bone revealed by solid-state NMR spectroscopy

Understanding the structure and structural changes of bone, a highly heterogeneous material with a complex hierarchical architecture, continues to be a significant challenge even for high-resolution solid-state NMR spectroscopy. While it is known that dehydration affects mechanical properties of bone by decreasing its strength and toughness, the underlying structural mechanism at the atomic level is unknown. Solid-state NMR spectroscopy, controlled dehydration, and H/D exchange were used for the first time to reveal the structural changes of an intact piece of bovine cortical bone. (1)H spectra were used to monitor the dehydration of the bone inside the rotor, and high-resolution (13)C chemical shift spectra obtained under magic-angle spinning were used evaluate the dehydration-induced conformational changes in the bone. The experiments revealed the slow denaturation of collagen due to dehydration while the trans-Xaa-Pro conformation in collagen remained unchanged. Our results suggest that glycosaminoglycans in the collagen fiber and mineral interface may chelate with a Ca(2+) ion present on the surface of the mineral through sulfate or carboxylate groups. These results provide insights into the role of water molecules in the bone structure and shed light on the relationship between the structure and mechanics of bone.

Stress mapping of undamaged, strained, and failed regions of bone using Raman spectroscopy

Stress differences via spectral shifts that arise among failed, strained, and undamaged regions of bone can be determined using Raman spectroscopy and double-notch specimens. A double-notch specimen is a model in which the early stages of fracture can be examined. At four-point bending, fracture occurs at one of the notches. Tissue near each notch is representative of bone in a state either directly before or after bone failure. Raman images are acquired among three regions: control, strained (root of unbroken notch), and failed (root of fractured notch). The center of gravities (CGs), a way to monitor wavenumber shifts, of the phosphate v(1) band are calculated. A PO(4) (-3) v(1) band shift most likely corresponds to a change in spacing between phosphate cations and anions. This spectral shift is converted into stress values using the dvdP coefficient, determined by applying known pressures/stresses and measuring the change in position of the PO(4) (-3) v(1) band. In comparison to control regions, the residual stress in strained and failed regions is significantly higher (p=0.0425 and p=0.0169, respectively). In strained regions, residual stress is concentrated near the corners of the unbroken notch, whereas in failed regions the high stresses are confined near the edge of the fracture.

Inbred strain-specific response to biglycan deficiency in the cortical bone of C57BL6/129 and C3H/He mice

Inbred strain-specific differences in mice exist in bone cross-sectional geometry, mechanical properties, and indices of bone formation. Inbred strain-specific responses to external stimuli also exist, but the role of background strain in response to genetic deletion is not fully understood. Biglycan (bgn) deficiency impacts bone through negative regulation of osteoblasts, resulting in extracellular matrix alterations and decreased mechanical properties. Because osteoblasts from C3H/He (C3H) mice are inherently more active versus osteoblasts from other inbred strains, and the bones of C3H mice are less responsive to other insults, it was hypothesized that C3H mice would be relatively more resistant to changes associated with bgn deficiency compared with C57BL6/129 (B6;129) mice. Changes in mRNA expression, tissue composition, mineral density, bone formation rate, cross-sectional geometry, and mechanical properties were studied at 8 and 11 wk of age in the tibias of male wildtype and bgn-deficient mice bred on B6;129 and C3H background strains. Bgn deficiency altered collagen cross-linking and gene expression and the amount and composition of mineral in vivo. In bgn's absence, changes in collagen were independent of mouse strain. Bgn-deficiency increased the amount of mineral in both strains, but changes in mineral composition, cross-sectional geometry, and mechanical properties were dependent on genetic background. Bgn deficiency influenced the amount and composition of bone in mice from both strains at 8 wk, but C3H mice were better able to maintain properties close to wildtype (WT) levels. By 11 wk, most properties from C3H knockout (KO) bones were equal to or greater than WT levels, whereas phenotypic differences persisted in B6;129 KO mice. This is the first study into mouse strain-specific changes in a small leucine-rich proteoglycan gene disruption model in properties across the bone hierarchy and is also one of the first to relate these changes to mechanical competence. This study supports the importance of genetic factors in determining the response to a gene deletion and defines biglycan's importance to collagen and mineral composition in vivo.

Raman spectroscopy of synovial fluid as a tool for diagnosing osteoarthritis

For many years, viscosity has been the primary method used by researchers in rheumatology to assess the physiochemical properties of synovial fluid in both normal and osteoarthritic patients. However, progress has been limited by the lack of methods that provide multiple layers of information, use small sample volumes, and are rapid. Raman spectroscopy was used to assess the biochemical composition of synovial fluid collected from 40 patients with clinical evidence of knee osteoarthritis (OA) at the time of elective surgical treatment. Severity of knee osteoarthritis was assessed by a radiologist using Kellgren/Lawrence (K/L) scores from knee joint x rays, while light microscopy and Raman spectroscopy were used to examine synovial fluid (SF) aspirates (2 to 10 microL), deposited on fused silica slides. We show that Raman bands used to describe protein secondary structure and content can be used to detect changes in synovial fluid from osteoarthritic patients. Several Raman band intensity ratios increased significantly in spectra collected from synovial fluid in patients with radiological evidence of moderate-to-severe osteoarthritis damage. These ratios can be used to provide a "yes/no" damage assessment. These studies provide evidence that Raman spectroscopy would be a suitable candidate in the evaluation of joint damage in knee osteoarthritis patients.